Geologic Setting of the Chena Hot Springs Geothermal System, Alaska

Abstract

A belt of moderate-temperature geothermal activityruns east-west across central Alaska. Chena HotSprings (CHS) is one of several Interior Alaskan hotsprings in or near 55 Ma granites within Paleozoicmetamorphic rocks. CHS is located within the CHSpluton, a composite body of quartz diorite, tonalite,granodiorite, and granite of both mid-Cretaceous andEarly Tertiary ages. 40Ar/39Ar step heat analyses ofbiotite from the CHS pluton shows flat Tertiary agespectra (supporting previous studies that reported aTertiary age for the body); however, hornblendespectra show evidence for partial resets and suggest amagmatic age of about 90 Ma. The majority of theexposed CHS pluton displays features common toCretaceous (subduction-related) plutons elsewhere ininterior Alaska. The pluton is medium to coarsegrained,commonly hornblende and allanite-bearing,with intermediate to felsic compositions and low Rb-Y-Nb-U-Th concentrations. Small bodies ofmineralogically and texturally distinct syenograniteand alkali feldspar granite occur near the margins ofthe composite body. These rocks contain up to 8times as much U and 4 times as much Th and Rb asthe typical rocks of the CHS pluton, as is true of earlyTertiary (extension-related) granites elsewhere inInterior Alaska. DIGHEM airborne gamma raysurveys indicate that high U-Th anomalies spatiallycorrelate with outcrops of these chemically distinctgranites. Thus, we propose that most of the exposedCHS pluton is Cretaceous and is underlain by anU,Th-enriched Tertiary pluton with limited surfaceextent. This hypothetical body reset the argonisotopic systematics of biotite and partially reset thehornblende. We further propose that radioactivedecay of K, U and Th within the concealed Tertiarypluton is causing an anomalously high regionalgeothermal gradient, thereby providing the heat atCHS. The hydrothermal upwelling zone may occur ata fault zone junction and/or at the hypotheticalcontact between Cretaceous and Tertiary plutons.